Lasers are used in measurement systems for the precise measuring associated with manufacturing processes. Precision distance measuring is critical to processes such as integrated circuit (IC) fabrication. With increased laser accuracy and precision, finer IC line widths can be used, resulting in higher circuit density for greater performance. Additionally, laser system precision increases accuracy in aligning multiple layers on ICs, resulting in higher yield.
A laser system has the benefit of repeatability, that is to say, the ability to repeat a precise measurement exactly. This benefit has been extremely important to the disk drive industry. Lasers allow for higher track density for greater data storage capacity, and also provide higher yields and greater throughput.
Machine tools and other precision cutting machines use laser transducers to produce more accurate parts with smoother surface finishes.
Typically in a laser measurement system, a quarter wave plate is used to change the polarization of the measurement beam in order that the beam makes two passes at the measurement mirror. During the passes, error may be introduced by air turbulence.
Air turbulence affects the performance of distance measuring interferometers such as He--Ne lasers in applications such as IC steppers or scan lithography systems. Typical length measurement error range between ten and thirty (10-30) nanometers over a twenty (20) centimeter path. This measurement error results in stage positioning errors which affect the achievable overlay accuracy. By using the dispersive characteristics of air, measurements of the optical path length at multiple wavelengths can provide the information required to remove the effects of air turbulence on the measured path length. The small optical path length difference at the two wavelengths is directly proportional to the integrated air density measurement path.
Air turbulence correction via two independent distance measurements (using DC interferometry) has been described by Matsumoto, et. al. (Appli. Optics, v.31 pp 4522-26, 1992) using a single frequency HeNe laser and a Nd:YAG laser. However, accuracy in this method is hampered by fringe counting error; and the requirement for and attendant expense of two lasers makes this solution prohibitively expensive.
To implement this method, the ratio of the two measurement wavelengths must be known exactly. The use of a fundamental wave and its second harmonic satisfies this requirement. What is wanted is a single laser source generating two overlapping beams at two harmonically-related wavelengths, both beams characterized with a high degree of accuracy. Further needed is air turbulence error correction. Ideally, such air turbulence error correction would be without the addition of bulk to the system or expense to the manufacture of the interferometer.
What is needed is a interferometer system that reduces error from air turbulence without significant additional bulk or cost.